1. Field of the Invention
This invention relates to data communication and particularly to a frequency shift keyed (FSK) detection apparatus in which a zero-crossing or change of sign is employed to determine signal state.
The output of a digital FSK detector is a sequence of binary values separated by an interval T. The binary serial information is retrieved by detecting a positive or negative value relative to a zero reference. For an audio band modulator-demodulator (modem), a reasonable value for the time interval T is approximately 130 microseconds. The interval value is generally chosen to minimize the processing power required of the particular filter and detection scheme. The value of the interval T determines the amount of uncertainty which may be tolerated between two successive samples during an interval in which a change of sign occurs. For example, at a K bit per second transmission of an FSK signal, the uncertainty in zero-crossing detection as a percentage of bit time is given by the expression: ##EQU1##
Where for example the interval T is equal to 130 microseconds and the transmission rate is K=1200 bits per second, the uncertainty in the time of zero-crossing is 15.6%. This is a relatively high uncertainty which contributes heavily to jitter, i.e., isochronous distortion, in the decoded signal.